Need to know: Born in South Africa to Greek parents, Retsos is making a name for himself in Germany with Bayer Leverkusen.

A versatile defender who has played at full-back despite being most comfortable as a centre-half, the 19-year-old is strong in the air and firm in the tackle, while he’s also willing and able to play out from the back.

The former Olympiakos man became the most expensive Greek player in history when Leverkusen splashed out €22m in the summer.

Highlight so far: Retsos appeared in October’s 2-1 win over Cyprus, a result which left Greece on the brink of reaching the play-offs for 2018 World Cup qualification.

19. Weston McKennie (Schalke)

Need to know: Christian Pulisic, John Brooks and Bobby Wood are among the United States internationals to have cut their teeth in the Bundesliga, and now McKennie is hoping to follow in his compatriots’ footsteps by stepping up to the senior national side in coming months.

A full-blooded holding midfielder, the 6ft 1in 19-year-old specialises in regaining possession for his team and moving the ball on simply to a team-mate.

McKennie has just earned his first full USA call-up for the forthcoming international against Portugal.

Highlight so far: McKennie made his first Bundesliga start against Bayern Munich in September, and inked a new five-year-deal at Schalke just eight days later.

18. Dayot Upamecano (RB Leipzig)

Need to know: “Dayot has everything needed to succeed in professional football,” said RB Leipzig defender Marvin Compper in June. It’s still early days for Upamecano’s career in Germany, but he’s already earned plenty of plaudits following his summer switch from Red Bull Salzburg.

Upamecano is an assured centre-back who reads the game well and is aggressive in the tackle, but the 19-year-old is also a tremendous passer. France aren’t exactly short of options in defence, but he’s an outside bet to make Les Bleus’ World Cup squad.

Highlight so far: The teenager showed maturity beyond his years in RB Leipzig’s impressive 3-2 defeat of Borussia Dortmund in October.

17. Ezequiel Barco (Independiente)

Need to know: A diminutive attacking midfielder capable of operating on the left flank or in the centre, Barco has established himself as Independiente’s go-to creator.

The 18-year-old Argentine is a fantastic dribbler whose two-footedness allows him to take on his opponent on either side, and he's adept at wriggling out of tight spaces with his skill, balance and agility.

Forward Barco has scored six goals in his club career up to now, including a confident finish against Deportes Iquique in the Copa Sudamericana. There's no doubt about his self-confidence, but also from his team-mates too – the teenager is already entrusted with penalty duties.

Highlight so far: Barco scored the equaliser in May’s 2-1 win against Huracan which helped Independiente qualify for the Copa Sudamericana.